Eppie, you asked for some velocity info.
I have a Winchester model 70 .308 with a factory 26" heavy barrel.
Federal GMM 168gr ammo averages 2700 fps from this rifle.
Following Unclenick's methods I arrived at 45.0gr of Varget with the same bullet which is at or near max in most manuals. This load is a bit faster at 2750 fps. It betters the accuracy of the GMM ammo by a small but repeatable margin. Experimentation continues...
The results from your rifle will probably be different.